To begin, you need to disassemble the Nerf Blaster, and take out the guts we don't need. Be careful to save the important parts, like the switches and motors. There are several teardown examples on YouTube with different approaches. I suggest reviewing a few of them to get a good idea what is necessary for this project.

1. Disassemble everything, and save all the parts, including screws, springs, switches, motors, and anything else that looks important.
2. Drill 3 holes in each side of the front of the blaster, just above the barrel, but not through the barrel. (see image below)
3. Drill a hole for the programming switch. I put it in the side of the case, but you can mount it wherever you want.  Just don't interfere with the mechanics of the firing mechanism. (see image below)
4. There may be some other mods to make to the firing mechanism based on what the YouTube videos say.  I actually let my son handle that part, so I didn't get any pictures of it.  But I think he had to remove a safety interlock somewhere.  Basically, if you don't see it in my remaining pictures, it probably shouldn't be there!

That should be all of the physical hacking necessary.  The rest is all circuits, wiring, and coding.